1. Field of the Invention
The present invention relates to a molding material which can produce cured products with excellent transparency and heat resistance comprising a polysilsesquioxane compound as a major component, a sealing material in which the molding material is used, and a sealed optical element comprising an optical element sealed by a cured product of the sealing material.
2. Description of Related Art
An optical element includes various lasers such as a laser diode (LD), a light emitting element such as a light emitting diode (LED), a light receiving element, a complex optical element, a photonic integrated circuit, and the like. In recent years, an optical element emitting a blue light or a white light with a short wavelength has been developed and widely used. A rapid increase in brightness of light emitting elements with a short wavelength of luminescence tends to accompany an increase of the calorific power of the optical element.
An optical element is usually used as a sealed optical element in which the optical element is sealed with a cured product of a sealing material.
In general, a composition containing a transparent epoxy resin as a major component which can produce a cured product with excellent heat resistance and the like has been known as the sealing material for optical elements.
However, along with an increase in brightness of optical elements in recent years, the cured products of the optical element sealing material are exposed to a higher energy light and a higher temperature generated by the optical elements which cause deterioration, resulting in cracking and yellowing (coloration) of the cured products. Cracks produced in the cured sealing material make the optical elements unusable. Coloration reduces transparency and lowers the brightness of the optical elements.
In order to solve these problems, sealing materials for optical elements using a polysilsesquioxane compound as a major component have been proposed in Patent Documents 1 to 3. The polysilsesquioxane compounds disclosed in Patent Documents 1 to 3 are claimed to produce a sealed optical element excelling in crack resistance and heat resistance.
However, it is sometimes difficult for the cured sealing materials for the optical elements containing the polysilsesquioxane compounds disclosed in Patent Documents 1 to 3 to sufficiently prevent yellowing when exposed to a high energy light or a high temperature generated by the optical elements. Therefore, development of a sealing material for optical elements which can produce a cured product exhibiting more excellent crack resistance and heat resistance has been desired.    [Patent Document 1] JP-A-2004-359933    [Patent Document 2] JP-A-2005-263869    [Patent Document 3] JP-A-2006-328231